This invention relates to power transistor devices, particularly but not exclusively of insulated-gate type, for example an insulated-gate power field-effect transistor (hereinafter termed MOSFET) or an insulated-gate bipolar transistor (hereinafter termed IGBT), wherein the device comprises a semiconductor body which accommodates an array of parallel device cells in which heat is generated in operation of the device, and wherein a hot-location temperature sensor is located inside the array of device cells.
United States patent specification U.S. Pat. No. 4,913,844 discloses such a device, wherein the device cells of an insulated-gate transistor type have a cellular region structure in the semiconductor body adjacent to a body surface at which a common source electrode and a common insulated-gate electrode are present. The hot-location sensor comprises at least one sensor cell which has a cellular region structure similar to that of the device cells. Its one or more sensor cells has/have an insulated-gate electrode which is coupled in common with that of the device cells and has/have a source electrode which is separate from the common source electrode of the device cells and which forms an output electrode that is coupled to a detection circuit.
For temperature sensing, U.S. Pat. No. 4,913,844 teaches the adoption of a current mirror arrangement of two such sensors inside the array. One of these hot-location sensors is used in a current sensing mode by coupling its source electrode to the device source terminal via a low value resistor, while the other hot-location sensor is used in a voltage sensing mode by coupling its source electrode to the device source terminal via a high value resistor. The temperature of the array is detected by calculating the on-resistance (from the source outputs of the voltage sensor and of the current sensor) and correlating the resistance value with the known temperature dependence of the on-resistance.
This is a complicated way to sense the temperature it also relies on the device cells and voltage sensing cells being operated in the saturation region of the device, in order that the voltage across the high value resistor equals (to a great degree of accuracy) the voltage on the drain of the voltage sensor and hence the voltage on the drain of the power device array. However, the saturation region is of less interest for monitoring excessive temperature increases within the array, because the power dissipation is low in the saturation region due to the very low voltage drop across the device (even though the current through the device is high).
United States patent specification U.S. Pat. No. 5,444,219 (our ref: PHB33667) discloses a temperature sensor in the form of a resistor, and also a differential temperature sensing circuit comprising such temperature-sensitive resistors at a hot location adjacent to the device array and at a cool location remote from the array. The hot-location and cool-location temperature-sensitive resistors are arranged in a Wheatstone bridge circuit in order to achieve sufficient sensitivity. This use of hot-location and cool-location temperature-sensitive resistors does not require the device cells to be operated in the saturation region. In a specific embodiment the hot-location temperature-sensitive resistor is outside the array at a distance of, for example, 125 .mu.m (micrometers) from the periphery of the array. However, it is suggested that the hot-location temperature-sensitive resistor may be positioned centrally within the array, although this arrangement will disrupt locally the regularity of the array. The whole contents of both U.S. Pat. No. 5,444,219 and U.S. Pat. No. 4,913,844 are hereby incorporated herein as reference material.